This invention relates to methanation of synthesis gas and, in particular, to a methanation assembly using multiple reactors for controlled methanation.
Methanation reactions have been well known for more than 70 years and have been applied in a variety of industrial processes, including ammonia synthesis, hydrogen plant purification and production of substitute natural gas from a variety of feedstocks. Methanation is generally used as a gas purification process to remove traces of carbon oxides from gases, such as synthesis gas (“syngas”). In addition, methanation has been used to produce synthetic natural gas (methane) and can be used to convert syngas to produce methane-containing syngas suitable for use in conventional fuel cell assemblies.
Particularly, methanation of syngas involves a reaction between carbon oxides, i.e. carbon monoxide and carbon dioxide, and hydrogen in the syngas to produce methane and water, as follows:CO+3H2→CH4+H2O   (1)CO2+4H2→CH4+2H2O   (2)
Methanation reactions (1) and (2) take place at around 300° C. in a methanation reactor filled with a nickel containing catalyst and are strongly exothermic. Generally, the temperature increase in a typical methanator gas composition used in a hydrogen plant is about 74° C. for each 1% of carbon monoxide converted and 60° C. for each 1% carbon dioxide converted.
As can be appreciated, because of the exothermic nature of methanation reactions (1) and (2), the temperature in the methanation reactor during methanation of syngas has to be controlled to prevent overheating of the reactor catalyst. Also high temperatures are undesirable from an equilibrium standpoint and reduce the amount of conversion of syngas to methane since methane formation is favored at lower temperatures. Formation of soot on the reactor catalyst is also a concern and requires the addition of water to the syngas. Further, in some types of fuel cell applications, the fuel cells require that the methane reforming be done in the fuel cell stack. The endothermic methane reforming heat promotes stack heat management, reduces fresh air requirements and improves overall power plant efficiency.
Accordingly, various conventional methods have been proposed in order to control the temperature of methanation reactions. In particular, conventional assemblies using multiple methanation reactors connected in series have been used in order to limit the temperature rise during exothermic methanation reactions. For example, U.S. Pat. No. 3,967,936 teaches the use of two or more methanation reaction zones connected in series and a plurality of quench zones situated between the reaction zones such that a mixture of feed gas and cold recycle gas is delivered to the quench zones for quenching of effluent gas that emerges from each of the reaction zones. In addition, U.S. Pat. No. 4,205,961 discloses a methanation assembly for methanation of synthesis gas including two high-temperature methanation reactors connected in series followed by two low-temperature methanation reactors also connected in series. In this case, boilers and heat exchangers fed with water are used to cool through heat exchange the effluent leaving one reactor and being fed to another reactor.
Conventional methanation methods have also used multiple methanation reactors connected in series and in parallel to process two or more feed gas streams. For example, U.S. Pat. No. 4,298,694 discloses a catalytic methanation process where a feed gas rich in carbon oxides is divided into two part streams, such that the first part stream is methanated in a first methanation reactor, and the cooled effluent from the first reactor is combined with the second part stream and is methanated in a second methanation reactor. The process disclosed in the '694 patent uses a heat exchanger supplied with saturated steam to cool the effluent exiting from the first reactor.
As can be appreciated, conventional methods of controlling temperature during methanation reactions require use of complex equipment including multiple heat exchangers and gas recycling components to cool the effluent gas and to prevent overheating of the methanation reactors. Additionally, conventional methanation methods have dealt with controlling soot formation on the methanation catalyst within the methanation reactor by adding steam to the reactants going to the first reactor.
It is therefore an object of the present invention to provide a methanation assembly using multiple methanation reactors with improved temperature control to produce a gas having a desired temperature and methane composition.
It is a further object of the present invention to provide a methanation assembly using multiple methanation reactors and direct water injection as a cooling medium to control the temperature in the methanation reactors as well as to avoid deposition of soot on the methanation catalyst.